The present invention relates to a vehicle travelling control apparatus for a vehicle with a hydrostatic transmission (hereinafter referred to as HST) and a mechanical transmission connected in tandem to the HST.
It is known from Japanese Unexamined Patent Application Publication (Kokai) No. H3-24366 for example that, in a vehicle with the HST and the mechanical transmission connected in tandem to the HST, the shifting operation of the mechanical transmission is automatically performed according to changes in load torque during vehicle travels.
The vehicle travelling control apparatus of the type disclosed in the above-cited publication is designed to automatically upshift and downshift the mechanical transmission according to changes in load torque due to various vehicle travelling conditions, so that a proper drive torque adapted to varying load torque in various travelling conditions can be obtained, while preventing a troublesome manual operation to be made for the gear-changing of the mechanical transmission in the conventional apparatus.
Specifically, the vehicle travelling control apparatus disclosed in the cited publication is designed to detect the hydraulic circuit pressure of the HST to downshift the mechanical transmission to a lower speed stage where the detected level is above a pre-set level for obtaining a higher drive torque, and upshift the same to a higher speed stage for a high speed travelling where the detected level is below the pre-set level.
The travelling control apparatus of the conventional type having the above arrangement omits the necessity of manual shift operation of the mechanical transmission for improvement of the operability of the vehicle. However, the apparatus of this type causes a great difference in speed due to shift-changes of the mechanical transmission. Hence, the ride quality of the vehicle may be deteriorated, and various adverse effects such as engine stall occur in some cases.
The present invention has been conceived to solve the above problems. It is an object of the present invention to provide a vehicle control apparatus for the vehicle with the HST and the mechanical transmission connected in tandem to the HST that is capable of automatically upshifting and downshifting the mechanical transmission according to varying load torque, while efficiently limiting speed variation of the vehicle due to the shift change of the mechanical transmission.
In consideration of the above cited prior art, according to the present invention, there is provided a vehicle travelling control apparatus for a vehicle with an HST and a mechanical transmission interposed in a travelling power transmission path between a driving power source and driving wheels. The vehicle travelling control apparatus includes a speed-change control mechanism which in turn includes a signal detection part and a control part for controlling the changing of the output speed of the HST and the shifting operation of the mechanical transmission. The signal detection part includes a load-torque detection means for detecting the load torque with respect to the vehicle. The control part is designed to control the mechanical transmission and the HST based upon the detected results by the load-torque detection means so that where the vehicle lies in a high load torque state, the control part downshift the mechanical transmission to a lower speed stage, while increasing the output speed of the HST.
According to the vehicle travelling control apparatus having the above arrangement, the mechanical transmission can automatically be downshifted to a lower speed stage in order to obtain a larger driving torque, where the vehicle lies in a high load torque state. It is also possible to effectively limit the variation of the vehicle speed due to the downshifting of the mechanical transmission.
The control part is preferably designed to control the mechanical transmission and the HST based upon the detected results by the load-torque detection means so that where the vehicle lies in a low load torque state, the control part upshifts the mechanical transmission to a higher speed stage, while decreasing the output speed of the HST.
With the thus designed control part, the mechanical transmission can automatically be upshifted to a higher speed stage in order to enable the vehicle to travel at higher speed, where the vehicle lies in a low load torque state. It is also possible to effectively limit the variation of the vehicle speed due to the upshifting of the mechanical transmission.
The vehicle travelling control apparatus having the above arrangement is preferably arranged in the following manner. Specifically, the HST includes a hydraulic pump and a hydraulic motor such as of the axial piston type and radial piston type connected to the hydraulic pump via a pair of hydraulic lines. The load-torque detection means is designed to detect whether the hydraulic pressure of the pair of hydraulic lines is above a reference level of high pressure side or below a reference level of low pressure side, thereby detecting which state out of the high load torque state, the low load torque state and a proper load torque state the vehicle lies in. The control part includes a memory for storing data concerning the relationship between the hydraulic pressure of the pair of hydraulic lines and the load torque of the HST, and a processor for calculating control signals transmitted to the HST and the mechanical transmission. The processor is designed to output control signals for the mechanical transmission and the HST based upon the detected signals from the load-torque detection means.
It is also preferable to employ the following arrangement. Specifically, the signal detection part includes a displacement-amount detection means for detecting the displacement amount of an output control member of the HST, and a lever pivoting angle detection means for detecting the pivoting angle of a operation lever designed to manually control the output control member of the HST. The memory is also designed to store data concerning a vehicle-speed-to-displacement-amount-relationship which is the relationship between the displacement amount of the output control member and the vehicle speed. The processor is designed to detect by using the displacement amount of the output control member inputted from the displacement-amount detection means a current vehicle speed based upon the vehicle-speed-to-displacement-amount-relationship in a currently engaging speed stage of the mechanical transmission, and determine by using the detected current vehicle speed whether the mechanical transmission lies in a state enabling its speed changing, based upon the vehicle-speed-to-displacement-amount-relationship in a speed stage of the mechanical transmission to be subsequently engaged.
The processor is preferably designed to control by using the detected current vehicle speed the output control member so as to prevent variation of the vehicle speed due to the shifting operation of the mechanical transmission, based upon the vehicle-speed-to-displacement-amount-relationship in a speed stage of the mechanical transmission to be subsequently engaged, where the processor has determined that the shifting operation of the mechanical transmission can be performed.
The vehicle travelling control apparatus preferably includes a hydraulic-pressure control mechanism for controlling the hydraulic pressure of working fluid for effecting engagement and disengagement of clutch devices in the mechanical transmission. The hydraulic-pressure control mechanism is disposed within a hydraulic circuit acting as a feeding passage of the working fluid, and includes a delay relief valve for gradually increasing the hydraulic pressure of the working fluid fed to a clutch device of the clutch devices to be subsequently engaged from an initial hydraulic pressure level to a preset hydraulic pressure level. The delay relief valve is designed so that the diameter of a throttle aperture for regulating the amount of the working fluid acting on the control piston is enlarged according to the increase of the hydraulic pressure in a hydraulic circuit of the HST.
With the above arrangement, it is possible to properly control the hydraulic pressure of the working fluid during it is gradually raised from the initial hydraulic pressure level to the preset hydraulic pressure level according to the load torque to the vehicle. That is, it is possible to perform the shifting operation of the mechanical transmission in a shorter period of time with a higher engaging torque, as the load torque to the vehicle increases. Thus, it is possible to prevent an abrupt speed change or shift shock at the time of shifting operation of the mechanical transmission and other undesirable effects, and perform the shifting operation at ideal timing without using a complicated, high-precision and expensive electronic control system, and hence improve the ride quality of the vehicle.
The delay relief valve is preferably designed so that a control piston moves a valve for effecting communication with and shutting off to a drain line from an initial hydraulic pressure position to a preset hydraulic pressure position, against the biasing force of a hydraulic pressure setting spring biasing the valve towards a side enabling the shutting off to the drain line, when the delay relieve valve gradually increases the hydraulic pressure of the working fluid from the initial hydraulic pressure level to the preset hydraulic pressure level. The delay relief valve is also designed so that the initial hydraulic pressure position of the control piston moves closer to the preset hydraulic pressure position according to the increase of the hydraulic pressure in a hydraulic circuit of the HST.
With the above arrangement, it is also possible to produce the effects as described above. Specifically, it is possible to perform the shifting operation of the mechanical transmission in a shorter period of time with a higher engaging torque, as the load torque to the vehicle increases. Thus, it is possible to prevent an abrupt speed or shift shock change at the time of shifting operation of the mechanical transmission and other undesirable effects, and perform the shifting operation at ideal timing without using a complicated, high-precision and expensive electronic control system, and hence improve the ride quality of the vehicle.
The delay relief valve is also preferably designed so that a control piston moves a valve for effecting communication with and shutting off to a drain line from an initial hydraulic pressure position to a preset hydraulic pressure position, against the biasing force of a hydraulic pressure setting spring biasing the valve towards a side enabling the shutting off to the drain line, when the delay relieve valve gradually increases the hydraulic pressure of the working fluid from the initial hydraulic pressure level to the preset hydraulic pressure level. The delay relief valve is also preferably designed so that the diameter of a throttle aperture for regulating the amount of the working fluid acting on the control piston is enlarged according to the increase of the hydraulic pressure in a hydraulic circuit of the HST. Moreover, the delay relief valve is preferably designed so that the initial hydraulic pressure position of the control piston moves closer to the preset hydraulic pressure position according to the increase of the hydraulic pressure in a hydraulic circuit in the HST.